1. Field of the Invention
This invention relates to a recycling fish rearing system and, more particularly to such a system including an ozone contacting unit for recycling water in which effluent water from a fish rearing tank can be treated with ozone and reused in the rearing tank.
2. Description of the Prior Art
There is an increasing demand for sport and commercial fishes. This demand can only be satisfied by means of hatcheries or similar operations. These fish rearing systems require large amounts of water to operate on an appropriately large scale. Waters have normally been drawn from rivers or lakes. Such waters may vary rapidly in quality and thus endanger fish stocks.
Reuse or recycling of water is becoming increasingly desirable because there is a diminishing water supply of suitable quality; risks to fish stocks occur due to the use of low quality water, variations in flow rate, solid content and disease levels. In addition pollution abatement costs are high for single pass systems.
In theory at least, reuse and recycling of water can provide: a substantial reduction in water requirements; optimization of fish growth rate by regulation of the water temperature; control of disease by sterilization of intake water; and reduction of overall water supply and pollution abatement costs.
The essential requirements of a water reconditioning system for a fish rearing system must include means to control the level of suspended solids such as fish feces and unused fish food, and ammonia and carbon dioxide. The water must not contain toxic levels of nitrite ion which is produced in many rearing systems and the water must be returned to the fish rearing tank of the system with sufficient dissolved oxygen for fish respiration.
Burrows et al in the Progressive Fish Culturist, 30: 123 to 136, 1968 described the use of a downflow flooded filter consisting of a 4 foot layer of half inch to 3 inch rock covered with a 1 foot layer of 1/4 to 3/4 inch oyster sheels. Unsettled hatchery effluent was applied at 0.83 gallons per minute per square foot of filter area. This unit has found practical application at the Dworshak Fish Hatchery in Oregon and has been installed by the Canadian Fisheries Service at the Capilano Hatchery in North Vancouver. However oyster shells were omitted from the latter application. Extensive operational problems have been encountered at Dworshak. These have been due principally to the blinding of the filters, to algae growth and to poor flow maintenance.
In Technical Report No. 67, New Mexico State University, 1970, 119 pp. Gigger et al described the use of filters consisting of 3/4 inch gravel or plastic media available under the trade mark SURFPAC in a semi-pilot scale test with 6 inch rainbow trout as the test species. Flow rate ranged from 3.5 to 5.34 gallons per minute per square foot and bed volumes were replaced at 1.39 to 4.19 minute intervals. The filter removed 150 to 200 mg. of ammonia nitrogen per hour per cubic foot of media. However the exposure time of fish in these tests was too short to assess long time ammonia toxicity effects. Furthermore no nitrate measurements were made on the test unit.
McCrimmon et al in the Progressive Fish Culturist 28: 165 to 170, 1966 described a laboratory scale water recirculation unit for holding trout. The unit involves sand filtration and carbon contacting of the recirculated water. Scott et al in Journal of the Fisheries Research Board of Canada 29: 1071 to 1074, 1974 showed that hatchery rearing water could be recycled by filtering through a two stage high rate filtration system using large particle anthracite coal. In their experiments the fish grew rapidly with negligible mortality. pH was maintained in the range of 6.5 to 7.5 but ammonia levels fluctuated widely in the range 4 to 22 milligrams of ammonia nitrogen per liter. Problems were also encountered with backwashing the filter.
Mayo et al, in "A Study for Development of Fish Hatchery Water Treatment Systems" prepared for Walla Walla District Corps of Engineers in co-operation with the U.S. Department of the Interior, U.S. Bureau of Sports, Fisheries and Wildlife, by Kramer, Chin and Mayo, Seattle, 1972, 42 pp., compared biological filter systems using one inch and three and one half inch Koch rings and one quarter inch foamed polystyrene (Styrofoam) pellets as the filter media. The systems were compared with biological systems designed on the activated sludge tank technique. The tests showed that all systems -- biological filter or activated sludge -- could be used for pollution control abatement but filter systems were more reliable when water reuse was a primary objective. Problems with nitrite toxicity occurred but these could be avoided by using controlled programs of management which would allow gradual increase in the fish load, thereby limiting the level of nitrite produced in the system.
Our co-pending United States application Ser. No. 573,548 entitled "A Fish Rearing System" describes a fish rearing system in which the fish rearing tank communicates with a primary filtration means to which water from the fish rearing tank is directed. The primary filtration is to remove particulate material rapidly. After the primary filtration means, the water passes to a biological filter of a particulate medium. This biological filter has a backwash means that includes an air blower whereby, during backwash, the filter medium is moved vigorously. In normal operation, that is when water is recycling, the water passes from the biological filter to an aeration unit where it is aerated and passed back to the rearing tank. During backwashing the water used in the backwashing is directed to waste. The system includes pump means to move water throughout the system and conduit means whereby water can be fed throughout the system.
The primary filtration means may be any filtration means able to remove a substantial proportion of particulate matter coming from the fish tank with low retention time of the water in the filtration means and, what is especially important, without blinding, that is blocking, of the filtration means. In general the primary filtration means should be able to remove up to 60% of the solids from the fish tank water in a dwell time in the filtration means of up to about 15 minutes. In a preferred embodiment described in our co-pending application the particulate medium of the biological filter comprises particles having a size in the range of 0.5 to 10 millimeters. Appropriate media include sand, granite, anthracite, glass beads or plastic beads having the above size. Typically a filter medium depth of about 30 inches has proved useful.
Using the above system, 76% conversion of ammonia by oxidation was obtained in the biological filter with hydraulic loads of 3570 gallons per square foot per day. The filter operated satisfactorily with ammonia loadings up to 0.002 pounds per cubic foot per day. The system gave substantially better results than any prior art system and is potentially capable of treating loadings of 0.004 pounds per cubic foot per day of ammonia nitrogen. However, problems with nitrite production become substantial at this loading level.
A review of the data obtained by investigators of hatchery reconditioning systems is presented in Table 1. The most significant feature of this data in relation to the present application is that measurable levels of nitrite ion were present in water to which fish were exposed in all the recycle units.
These systems are often subject to nitrite toxity problems. Moreover, nitrite toxemia is a major problem occurring in conventional recycle systems for fish rearing. The only presently available course by which nitrite toxicity may be avoided by operators of recycle hatchery systems is to carefully regulate fish loads so that nitrite levels are similar to those indicated in Table 1 (0.02 to 0.10 mg NO.sub.2 --N/l).
The presence of nitrite ion in water is known to produce changes in fish blood methemoglobin content. This reduces the blood oxygen transport capacity and hence lowers the fish's swimming ability and resistance to shock or desease. Tests carried out by Brown and McLeay "Effect of Nitrite Concentration on Acute Toxicity, Methemoglobin and Total Hemoglobin levels in Juvenile Rainbow Trout (Salmo gairdneri)". Prepared for the British Columbia Department of Public Works of Recreation and Conservation, Fish and Wildlife Branch, by B. C. Research, Vancouver, 1974, showed that the methemoglobin content of rainbow trout blood was increased significantly by nitrite levels as low as 0.015 mg NO.sub.2 --N/l and the 96-hour LC50 (i.e. the concentration producing 50% mortality at 96 hr) was 0.230 mg NO.sub.2 --N/l.
TABLE 1 __________________________________________________________________________ COMPARISON OF FISH REARING SYSTEMS Unit Trickling Upflow Upflow Downflow downflow Activated Extended Our Co-Pending Type Filter* Filter* Filter* Filter+ Filter+ Sludge* Aeration* Applicaton (FIG. __________________________________________________________________________ 2) Hydraulic load gpm/ft.sup.2 2.0 2.1 1.7 0.75 0.72 -- -- 1.64 to 2.48 Retention Time (hr) Filter or aeration tank 0.294 0.236 0.265 0.47 0.43 0.37 1.46 0.1 to 0.16 Total system 1.2 0.58 0.83 0.93 1.2 1.2 to 1.7 Filter depth (feet) 4 4 4 4 4 2.5 Media type Koch Rings Koch Rings Styrofoam Osyter Gravel Sand Media size (inch) 3.5 3.5 0.25 to 0.5 0.25 to 3.0 0.25-1.5 0.04 Volume to treat 1 mgd 1,390 1,323 1,635 3,707 3,861 701 to 1,059 (cu ft) Ammonia removal (%) 30 19.7 33.8 33.2 20.9 poor poor 76 Ammonia in contact with 0.03 to fish** mg NH.sub.3 -N/1 0.1 to 1.26 0.13 to 2.4 0.17 to 2.33 -- -- 0.18 to 2.56 3.150 0.015 to 0.18 Nitrite in contact with 0.01 to fish** mg NO.sub.2 -N/l 0.03 to 0.07 0.05 to 0.11 0.06 to 0.11 -- -- 0.001 to 0.73 1.83 0.02 to __________________________________________________________________________ 0.09 *Mayo et al +Burrows type filters **During good operation